Field of the Invention
The present application relates to safety pressure limiting features on cryogen vessels, particularly in respect of cryogen vessels containing superconducting magnets of magnetic resonance imaging (MRI) systems. In particular, it relates to the advantageous arrangement of components of an auxiliary vent path, provided to limit pressure within the cryogen vessel in case of a quench of the superconducting magnet.
Description of the Prior Art
FIG. 1 shows a conventional arrangement of a cooled superconducting magnet 10 within cryogen vessel 12, itself retained within an outer vacuum chamber (OVC) 14. One or more thermal radiation shields 16 are provided in the vacuum space between the cryogen vessel 12 and the outer vacuum chamber 14. In some known arrangements, a refrigerator 17 is mounted in a refrigerator sock 15 located in a turret 18 provided for the purpose, towards the side of the cryostat. Alternatively, a refrigerator may be located within an access turret 19, which retains access neck (vent tube) 20 mounted at the top of the cryostat. The refrigerator provides active refrigeration to cool cryogen gas, typically helium, within the cryogen vessel 12, in some arrangements by recondensing it into a liquid 22. The refrigerator may also serve to cool the radiation shield 16. As illustrated in FIG. 1, the refrigerator 17 may be a two-stage refrigerator. A first cooling stage is thermally linked to the radiation shield 16, and provides cooling to a first temperature, typically in the region of 80-100K. A second cooling stage provides cooling of the cryogen gas to a much lower temperature, typically in the region of 4-10K.
A negative electrical connection 21a is usually provided to the magnet 10 through the body of the cryostat. A positive electrical connection 21 is usually provided by a conductor passing through the vent tube 20.
For fixed current lead (FCL) designs, a separate vent path (auxiliary vent) (not shown in FIG. 1) is provided as a fail-safe vent in case of blockage of the vent tube 20. It is this auxiliary vent path which is the subject of the present invention.
FIG. 2 shows an example of a conventional over-pressure limiting protection arrangement 30, designed to vent cryogen gas from the cryogen vessel in case of over-pressure, such as could occur following a quench of the magnet.
A superconducting magnet 10 is contained within a cryogen vessel 12 as discussed with reference to FIG. 1. A turret outer assembly 24 encloses upper extremities of access neck (vent tube) 20 and positive current lead 21, and provides a normal exit path 26 for cryogen gas from cryogen vessel 12. Turret outer assembly 24 is joined to the cryogen vessel in a leak-tight manner and defines an interior volume which is separated from atmosphere by a protective valve and/or burst disc, and forms part of normal exit path 26. The protective valve and/or burst disc in the illustrated example is quench valve 32.
In the event of a quench, the cryogen vessel 12 is vented to atmosphere via the vent tube 20 in the access turret 19 through the interior volume of the turret outer assembly 24 and quench valve 32. Quench valve 32 includes a valve plate 34 which is held against valve seat 36 by a spring arrangement 38. Cryogen egress tube 40 leads exit path 26 to atmosphere, or to a cryogen recuperation facility, essentially at atmospheric temperature. In case of over-pressure within cryogen vessel 12, a corresponding pressure of cryogen gas within the turret outer assembly 24 acting on the inner side 34a of the valve plate 34 will exceed the pressure acting on the outer side 34b of the valve plate sufficiently to overcome the force of the spring arrangement 38 and open the valve 32. Cryogen gases will escape, maintaining the pressure within the cryogen vessel at an acceptable level. Once the pressure in the cryogen vessel and the interior volume of the turret outer assembly 24 drops below the pressure needed to keep the quench valve 32 open, spring 38 will press the valve plate 34 back into contact with valve seat 36.
Part of the valve plate 34 may be formed by a burst disc, not visible in the drawing as it lies in the plane of the valve plate 34. In case the differential pressure across the valve plate becomes much higher than the pressure at which the quench valve 32 should open, for example if the quench valve 32 sticks, or the pressure increase within the cryogen vessel is extremely rapid or severe, the burst disc will rupture and cryogen gas will then escape through a hole left by the burst disc and out of the cryogen vessel 12 through the interior volume of the turret outer assembly 24 and egress tube 40. This burst disc is typically a declared regulatory pressure relief safety device, provided to rupture in the event of quench valve failure.
In addition to the declared safety device, an auxiliary vent path 42 is provided, through a tubular positive current lead 21 to atmosphere via an external room-temperature tube 44 fitted with its own auxiliary burst disc 46. Auxiliary vent path 42 does not pass through the interior volume of the turret outer assembly 24. The auxiliary burst disc 46 is designed to rupture when a differential pressure across it meets a certain value, in excess of the differential pressure at which quench valve 32 is designed to open, and in excess of the differential pressure at which the bust disc within valve plate 34 is designed rupture.
It is known that air ingress into the access neck 20 may cause ice to form in region 48, between the inner wall of the access neck 20 and the positive current lead 21. If sufficient ice forms in this region, it may form a constriction, and cryogen gas may not be able to freely escape in case of a quench. A differential pressure may exist across the blockage, reducing the differential pressure across the quench valve 32.
On the other hand, the positive current lead 21 passes into the cryogen vessel more deeply than the ice-forming region 48, to the level of temperatures usually so cold that any air ingress into the access neck 20 freezes onto the access neck in region 48 and before it can reach the lower end of the positive current lead 21. The interior of the tubular positive current lead 21 may therefore be assumed to be free of ice. As there is no blockage in the positive current lead, the full differential pressure between the interior of the cryogen vessel 12 and atmospheric pressure in the egress tube 40 will apply across the auxiliary burst disc 46. Burst disc 46 is designed to rupture at a pressure high enough that it can only be reached if the quench valve 32 and its burst disc have failed to protect the cryogen vessel as designed.
Typically, quench valve 32 is designed to open in response to a 0.5 BAR (50 kPa) differential pressure between the high pressure side 34a exposed to the interior volume of the turret outer assembly 24 and the low pressure side 34b exposed to the interior of the egress tube 40. The burst disc within the quench valve is typically designed to rupture in response to a differential pressure of 1.4 BAR (140 kPa), and the auxiliary burst disc 46 is typically designed to rupture in response to a differential pressure of 1.8 BAR (180 kPa). These values are chosen to protect the cryogen vessel in all circumstances, but are sufficiently separated that the quench valve 32 will open without damage to the burst disc within the quench valve unless the quench valve is stuck, and that the auxiliary burst disc 46 will only rupture in response to a cryogen vessel pressure so high that it is clear that neither the quench valve 32 nor the burst disc within the quench valve are going to open.
This arrangement has certain drawbacks, which the present invention seeks to alleviate.
In present arrangements such as shown in FIG. 2, the auxiliary burst disc 46 is permanently subjected to the full differential pressure between the interior volume of the turret outer assembly 24 and the cryogen vessel on one side and the egress path 40, which is at approximately atmospheric pressure, on the other side. This differential pressure may approach the pressure at which the auxiliary burst disc 46 is designed to rupture.
During a quench event which is vented through the auxiliary burst disc 46, the pressure within the cryogen vessel may approach the maximum allowable working pressure of the cryogen vessel, due to the constriction of escaping gas in the “room-temperature” tube 44 and the rapid expansion of this cryogen gas due to heating as it passes through the “room temperature” tube 44. It would be preferable from this point of view to provide a room temperature tube 44 of increased cross-section, but this would have the undesired effect of increasing the height of the overall system.
In the event of rupture of the auxiliary burst disc 46, air can be drawn back into the auxiliary vent path 42 once the over-pressure within the cryogen vessel has ceased. This can cause a buildup of ice within the tubular positive current lead 21 which is difficult to detect or remove.
A further disadvantage is the cost of the external room-temperature pipe work 44 and seals required to interface the auxiliary vent path 42 to the remainder of the equipment. The external pipe work 44 adds to overall system height, which causes integration problems in siting the cryostat. Any external joints, seals, welds etc. all have the potential to cause leaks into the vent path during normal service, and so their number should preferably be reduced.